1. Field of the Invention
This invention relates to a method of calibrating a probe for a coordinate positioning machine.
2. Description of Related Art
Coordinate positioning machines, such as coordinate measuring machines, machine tools and articulating measuring arms, comprise an arm and a table moveable relative to each other. Such a machine may be equipped with a probe to enable inspection of a workpiece. A probe may be typically classed as digital or analogue.
To enable use of the probe as an inspection device it must first be calibrated. A first aspect of probe calibration is the identification of the probe contacting tip in relation to the machine on which it is mounted. This may be called datuming or qualification. During the inspection process, the machine onto which the probe is mounted is driven towards the workpiece.
In the case of digital probes, contact with a surface causes a change in the state of the probe signal (from 0 to 1 or vice versa), a trigger signal is emitted and the machine outputs are latched. This latched position of the machine along with tip qualification information can be used to identify the position of a workpiece.
In analogue probing systems, a stylus is deflected when its tip contacts the workpiece and measurement transducers in the probe generate outputs representing the deflection of the stylus along three orthogonal axes which may be referred to as the a,b,c axes of the probe. These outputs, in addition to the tip qualification can be used to identify the position of a workpiece. When contact with a surface occurs, the probe signal changes, for example, gradually increasing in value. A contact is deemed to have occurred once the signal passes a threshold. A threshold is set to minimise the effect of false triggering due to machine vibrations. In order to establish where contact occurred, the change in signal strength must be related to movement of the machine axes. One way to do this is to calibrate the system by monitoring voltage output of the probe and relating the voltage output to movement along the three machine axes by use of a calibration matrix.
Thus, in the case of digital probes, all that is required is a tip qualification or datuming process. For analogue probes however, the output signal of the probe must be related to movement of the machine axes which requires a further calibration process. This calibration process involves both a datuming step and resolving the calibration matrix. This forms a second aspect of probe calibration.
One method of qualifying or datuming is to contact a sphere, which is mounted on the machine, with the stylus at least four different circumferential locations from which the stylus tip centre is established.
One method of performing an analogue probe calibration is described in International Patent Application No. WO00/25087 where a calibration artefact is mounted on the machine. The probe is driven towards the artefact, for example, along one of the machine axes, until an increase in the output of the measuring devices of the probe above a predetermined level (the threshold) indicates that contact with the artefact surface has occurred. After stylus contact has been confirmed, a set of machine x,y,z and probe a,b,c coordinate data are taken. Machine movement continues until the machine has moved a selected distance beyond the confirmed contact point, and a further set of x,y,z and a,b,c data are taken.
The changes in the a,b,c outputs of the probe's measurement transducers in the three axes are recorded and correlated with the changes in the machine's measurement devices along each of the machine axes. This is repeated for two other orthogonal directions, which may be the other two machine axes. From the sets of data, a probe transformation matrix can be established which relates the probe outputs in the a,b,c axes to the machine's x,y,z coordinate system for that particular probe orientation. The relevant machine axis components of the probe deflections can be obtained by multiplying the relevant probe output by the relevant matrix term.
An alternative method of performing an analogue probe calibration is described in International Patent Application No. WO02/073128.
It is often desirable to re-orientate the probe relative to the machine to enable inspection of differently oriented surfaces of the workpiece. The probe may be oriented on a probe head which may be an indexing type where it can be re-orientated into a number of discrete positions or, a continuous type where any angular orientation is possible.
There are a number of factors which affect the accuracy of a measurement made by a re-orientatable probe. These include machine errors such as squareness of axes, bending and linear errors; probe head errors including bending and positioning; and probe and stylus errors including bending. A bending error may be partially caused by gravity and partially caused by dynamic forces such as acceleration.
Traditionally, when indexing a measurement probe, it must be calibrated for each orientation of the probe relative to the machine axes to establish the true location of the stylus tip i.e. a re-datuming or re-qualification process is carried out. Furthermore, if the probe is an analogue transducer, the calibration process described above where a calibration matrix is established for a particular probe orientation along with a datuming step, must be carried out for each probe head position (or orientation). As it takes a few minutes to complete the process to obtain one such matrix, the entire calibration process can take hours to complete.
In European Patent No. 759534, the re-datuming step is simplified by inferring the current tip position using datum data from two positions between which the current position lies. Although the process is simplified, a plurality of datum readings must still be taken using this method.
Alternatively, for continuous probe heads, an encoder may be provided which provides the actual angular position of the probe. However, the relationship between the probe and the stylus must still be established. Thus, the probe is still datumed at each orientation or a plurality (if inferring is carried out) of orientations.
The datuming step described above applies to styli of the type where deflection is sensed at a location remote from the tip, for example by strain gauges or optical means. However, for contact probe systems where the tip position is transduced at the tip, it is only necessary to carry out tip qualification at one orientation as the tip location is constantly monitored. For this second type of stylus, used with an analogue probe, a calibration matrix must still be established for each orientation.
A third aspect of probe calibration is the detection and correction of inertial errors associated with the motion of a metrology system. It is known to compensate for such dynamic errors by providing an accelerometer in the probe from which can be determined dynamic deflection. Examples of such compensation methods are disclosed in U.S. Pat. Nos. 4,333,238 and 6,412,329.